Method and apparatus to dry powdery substances under a vacuum using electrical gas discharges



March 1966 G VOiGTLAENDER-TETZNER 3,238,632

METHOD AND APPARATUS TO DRY POWDERY SUBSTANCES UNDER A VACUUM USINGELECTRICAL GAS DISCHARGES Filed Aug. 30, 1962 2. Sheets-Sheet 1 Kamzaomwzk. I .v 5mm...

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GERHARD VOlGTLAENDER-TETZNER ATTORNEYS GAS WITH DRAWA L March 1966 G.VOlGTLAENDER-TETZNER 3,

METHOD AND APPARATUS T0 DRY POWDERY SUBSTANCES UNDER A VACUUM USINGELECTRICAL GAS DISCHARGES Filed Aug. 30, 1962 2 Sheets-Sheet 2 Torr sec

300 min I I L BY: 40.4404 7 W ATTORNEYS United States Patent 3,238,632METHOD AND APPARATUS T0 DRY POWDERY SUBSTANCES UNDER A VACUUM USINGELEC- TRICAL GAS DISCHARGES Gerhard Voigtlaeuder-Tetzner,Leverkuseu-Schlebusch,

Germany, assignor, by mesne assignments, to Leybold- Anlagen HoldingA.G., Zug, Switzerland Filed Aug. 30, 1962, Ser. No. 220,454 Claimspriority, application Germany, Aug. 30, 1961, L 39,906 12 Claims. (Cl.34-1) The present invention relates generally to a method for vacuumdrying or vacuum degassing substances in powder form, and especiallysubstances having a small average grain size, preferably below cm.

Considerable difficulties are encountered when drying and/ or degassingsubstances in powder form having small grain sizes. The reason for thisis that during the process there is a decrease in the size of the emptyhollow spaces and cavities between powder particles and, therefore,circulation of the molecules, which is necessary for gas and/or vaporexchange, also decreases. In industrial processes, powered substanceshaving very fine grain size including grain sizes which are less than10"*, are used as a heat insulating material. However, in order toobtain the extremely small heat conductivity which is possible withpowdered materials of the above-mentioned type, drying and degassingprocesses must be performed on the material. The vacuum which is usedfor this purpose and which, under some circumstances, is a high vacuum,can sometimes also be maintained in the container in which the body ofpowder is housed.

In the drying and degassing of small particles there is an undesiredinteraction between the individual particles which increases as thegrain size decreases. When a vacuum is provided and accompanied byheating, the gaseous surface layers which surround the particles areremoved and the particles can then contact each other and strongmolecular surface forces become effective. By means of these forces, theparticles adhere to each other and/or to the walls of the receptacle orcontainer. Due to this interaction agglomerations of the material areoften formed within the powdery contents of the container and these areof such strength or hardness that they cannot be separated even whenusing powerful mechanical means, such as by pounding with iron rods.

As mentioned above, the cause of this very undesirable effect is theformation of surface forces which increase to extreme extents when theparticles approach intimate contact with each other. A similar effect isused in technical fields in slip gauges. In such devices the sectionsadhere to each other tightly when the surfaces contact one another. Thereason for this is that the surfaces are very uniform or smooth and thecontacting and adhering is due to molecular adhesion forces.

With these defects of the prior art in mind, it is a main object of thepresent invention to provide a method of drying or degassing powderysubstances in a manner which is much improved over the prior art.

Another object of the present invention is to provide a method of thecharacter described wherein agglomerations of powdery substances underthe effect of surface forces are prevented during drying or degassingprocesses.

These objects and others ancillary thereto are accomplished according topreferred embodiments of the invention wherein a body of powderysubstance is provided and is subjected to the influence of a vacuum.During the period it is subjected to the vacuum, an electrical gasdischarge is created i the body of powdery material. As used in thefollowing specification and claims, the term gas discharge has theconventional meaning of a discharge 3,238,632 Patented Mar. 8, 1 966formed under reduced pressure by the application of a break-down voltagebetween a pair of electrodes. The discharge paths thereby formed createchannels in the powder layers and thus loosen the powder particles. Byproperly arranging the discharge electrodes, the electrode dischargeprocesses can be caused to occur to the greatest degree in certainpreferred portions of the body of powdery material. However, in manyinstances a uniform effect is desired, i.e., an effect of the electricaldischarge upon the entire powdery body is desired and may be uniformthroughout.

The gas discharge may be caused by a high frequency electric alternatingfield which may preferably be generated with the aid of a known Teslatransformer or coil. This type of discharge causes only a little heat tobe generated in the powdery substance and this heat also contributes tothe acceleration of the drying and degassing processes.

Instead of using the irregular spark discharges, glow discharges of lowluminous flow may be used within the powder layer. However, the greatermechanical forces provided by the spark discharge are generally moreadvantageous. Since, in the instant invention, an electrical gasdischarge is to be maintained in the body of powdery material, it shouldbe noted that at least during a portion of the drying and degassingprocess the pressure within the receiving container must not be loweredto the extent that the electrical discharge will be extinguished.

Additional objects and advantages of the present invention will becomeapparent upon consideration of the following description when taken inconjunction with the accompanying drawings in which:

FIGURE 1 is a schematic view of an arrangement for degassing fine grainpowders.

FIGURE 2 is a graphical view illustrating the relative gas withdrawal offine grain powders according to the present invention and according toprior art processes.

FIGURE 1 illustrates a vessel or receiving container 1 in which a bodyof fine grain powdery material 2 is disposed. This powdery material canbe formed of Aerosil powder having a medium grain size of 10- cm. Aheating coil 3 is disposed around the outer wall of the container 1 andthis coil may be activated or heated by connected it with a currentsupply.

A discharge probe 4 extends into the powdery body of material 2 and isconnected at one end to a Tesla transformer 6 by means of a connectionline 5. The other pole of the Tesla transformer 6 is grounded by meansof connection line 7. The inner wall of the container 1 acts as acounter electrode for the discharge probe 4, and is also grounded bymeans of a line 8.

The receiving container 1 is sealed by closure element 11 and isconnected to a vacuum pump 10 by means of an evacuating line or conduit9. A pressure measuring device 13 of a known construction, for example,an ionization manometer, is connected to the evacuating line 9. Also aventilating valve 14 is connected to the evacuating line 9 and both ofthese connections are provided by means of a cross piece 12 formed ofconduit material. A filter 15 is disposed in the evacuating line 9 andin the flow path and upstream of, or before, the cross piece 12 andprevents the fine grained powder from entering the measuring device 13and the pump 1t). Downstream or behind the cross piece 12 and in theevacuating line 9 is a sealing or blocking valve 16.

During operation of the device the interior of container 1 is evacuatedby means of line 9 and the vacuum pump 10, with the valve 16 being open.The amount of pressure which is provided can be determined by resortingto the pressure measuring device 13. At this point the ventilating valve14 remains closed. Also, the body of powdery material is heated by meansof coil 3 and elec- Torr liters as the abscissa seconds where 1 TO1T=1mm. of Hg, and pumping time in min utes as the ordinate. Curve aillustrates the nature of the gas withdrawal without the benefit of theelfect of an additional electrical discharge. However, curve billustrates the more advantageous conditions which are provided whensuch a discharge is simultaneously provided during the vacuum operation.

In this example the Aerosil powder had a medium or average grain size of10' cm. and an initial moisture content of 1.5%. The graph clearly showsthat a desired high value of gas withdrawal of the powder can beobtained with the method according to the present inven tion. In otherwords, a more extensive degassing of the body of powdery material isprovided than is possible with evacuation or operation without thebenefit of this discharge effect. From practical experiments it is seenthat the vacuum treatment of the body of powdery material, when gasdischarge is carried out simultaneously therewith, is degassed about tentimes as much as is provided without this auxiliary measure. It isessential that such a strong degassing can be accomplished with minimalheating of the powdery body.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes, andadaptations, and the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

What is claimed is:

1. A method for vacuum treatment of powdery substances having smallaverage grain size of below 10- cm. comprising the steps of depositing abody of powdery material in an evacuable chamber, subjecting the body ofpowdery material to a vacuum, and, at least during a portion of the timethat it is subjected to the vacuum, creating an electrical gas dischargethrough the interior portions of the body of powdery material so as tocause agitation thereof.

2. Apparatus for vacuum treatment of powdery substances, comprising, incombination:

(a) a vacuum tight chamber for receiving powdery substances;

(b) vacuum pump means for producing a vacuum connected to the interiorof said chamber; and

() gas discharge producing means comprising an electrode disposed withinsaid chamber and a power supply for applying a break-down voltage tosaid electrode, said gas discharge producing means adapted to produce anagitating gas discharge through the interior portions of a body ofpowdery substance in said chamber while a vaccum is produced therein.

3. The apparatus according to claim 2 wherein said electrode is disposedwithin saidchamberso as to project into the interior portion of the bodyof powdery substance.

4. The apparatus according to claim 3 wherein said vacuum tight chamberis a grounded electrically conductive container and provides a dischargepath for the gas discharge produced by said gas discharge producingmeans.

5. The apparatus according to claim 2 wherein said gas dischargeproducing means is adapted to produce a spark discharge at saidelectrode disposed within said chamber.

6. The apparatus according to claim 5 wherein said electrode is disposedwithin said chamber so as to project into the interior portions of thebody of powdery substance.

7. The apparatus according to claim 5 wherein said gas dischargeproducing means includes a Tesla transformer device adapted to produce aspark discharge.

8. The apparatus according to claim 2 including, in addition to said gasdischarge producing means, heating means adapted to produce heating ofthe body of powdery substance.

9. The apparatus according to claim 8 wherein said electrode is disposedwithin said chamber so as to project into the interior portions of thebody of powdery substance.

10. A method for vacuum treating an average powdery substance having agrain size below 10 cm., comprising the steps of depositing the powderysubstance in a vacuum chamber so as to at least partially bury anelectrode disposed within said vacuum chamber, evacuating said vacuumchamber, and producing a gas discharge at said electrode so as to causeagitation of the powdery substance during said evacuation step.

11. The method defined in claim 10 wherein the gas discharge produced isa spark discharge.

12. The method defined in claim 10 wherein the gas discharge produced isa glow discharge.

References Cited by the Examiner UNITED STATES PATENTS 773,875 11/1904Lorillard 34l5 773,876 11/1904 Lorillard 34-15 X 2,038,251 4/ 1936 Vogt.2,442,114 6/1948 Brown 34-1 2,585,825 2/1952 Nyrop 341 2,605,554 8/1952Flosdorf 34-1 3,011,543 12/1961 McCormick 159-4 3,060,297 10/ 1962Sargent.

FOREIGN PATENTS 898,989 5/ 1945 France.

704,491 2/1954 Great Britain.

911,885 11/1962 Great Britain.

WILLIAM F. ODEA, Primary Examiner.

GEORGE D. MITCHELL, ROBERT A. OLEARY,

MEYER PERLIN, Examiners.

J. SOFER, W. E. WAYNER, Assistant Examiners.

1. A METHOD FOR VACUUM TREATMENT OF POWDERY SUBSTANCES HAVING SMALLAVERAGE GRAIN SIZE OF BELOW 10**-4 CM. COMPRISING THE STEPS OFDEPOSITING A BODY OF POWDERY MATERIAL IN AN EVACUABLE CHAMBER,SUBJECTING THE BODY OF POWDERY MATERIAL TO A VACUUM, AND, AT LEASTDURING A PORTION OF THE TIME THAT IT IS SUBJECTED TO THE VACUUM,CREATING AN ELECTRICAL GAS DISCHARGE THROUGH THE INTERIOR PORTIONS OFTHE BODY OF POWDERY MATERIAL SO AS TO CAUSE AGITATION THEREOF.